1. Technical Field
The present invention relates to a technology of a charging roller having ring-like gap members or gap members composed of tape-like film members fixed to both end portions thereof to form a predetermined charge gap relative to an image carrier so that the charging roller charges the image carrier in non-contact state. The present invention also relates to a technology of an image forming apparatus, composed of an electrophotographic apparatus such as an electrostatic copying machine, a printer, and a facsimile, provided with the charging roller.
2. Related Art
As examples of image forming apparatuses, image forming apparatuses each provided with a charging roller which has a predetermined charge gap relative to an image carrier so as to conduct non-contact charging of the image carrier have been known by JP-A-2001-296723 (hereinafter, referred to as Document 1) and JP-A-2004-109151 (hereinafter, referred to as Document 2). As shown in FIG. 34A, a charging roller “a” used for each of image forming apparatuses respectively disclosed in Document 1 and Document 2 comprises a metal core “b” and a resistive layer “c” covering the peripheral surface of the metal core “b”. The resistive layer “c” is composed of an elastic member having conductive property. On the peripheral surfaces of the both end portions of the resistive layer “c”, a pair of gap members “d”, “e” which are composed of tape-like film members having insulation properties are wrapped into ring-like shapes and fixed or a pair of ring-like gap members “d”, “e” having insulation properties are fixed. The gap members “d”, “e” are brought in contact with the peripheral surface of a photoconductive drum “f” as an image carrier, whereby a predetermined charge gap G is defined. In this case, respective bearings “i”, “j” of rotary shafts “g”, “h” which coaxially extend from the ends of the metal core “b” are pressed toward the photoconductive drum “f” by biasing force of compression spring “k”, “m”, thereby bringing the gap members “d”, “e” in contact with the peripheral surface of the photoconductive drum “f” with some pressure.
Non-contact charging of the photoconductive drum “f” achieved by the charging roller “a” through the charge gap G produces less ozone. Further, the non-contact charging prevents foreign matter such as toner particles adhering to the photoconductive drum “f” from adhering to the charging roller “a” and also prevents substances contained in the resistive layer “c” of the charging roller “a” from adhering to the photoconductive drum “f”, thereby improving the chargeability of the photoconductive drum “f” by the charging roller “a”.
Generally, a driving gear fixed to the rotary shaft of the metal core “b” is connected to a driving gear fixed to the rotary shaft of the photoconductive drum “f” via a power transmission gear train, but not shown, so that driving force from the motor is transmitted to the driving gear for the charging roller “a” via the driving gear of the photoconductive drum “f” and the power transmission gear train, thereby rotating the charging roller “a”.
By the way, in the charging roller disclosed in Document 1, when the tape-like film member is wrapped around the charging roller, a joint portion is generated between an end and the other end of the film member. On the other hand, to constantly obtain stable charge on the image carrier, the charge gap G must be always kept constant at any position in any direction when the charging roller “a” is rotated. For this, the tape-like film members as the gap members “d”, “e” are required to be wrapped around the charging roller “a” not to generate a space between the both ends of each film member (both ends in the circumferential direction of the charging roller “a”) and not to superpose the both ends on each other in the vertical direction (the radial direction of the charging roller “a”). However, to achieve such wrapping of the film member around the charging roller “a”, it is required not only to set the length of the film member with exquisite precision but also to wrap the film member to the charging roller “a” with exquisite precision. Accordingly, it is required to carry out extremely strict dimensional control of the film members, thus deteriorating the productivity and also increasing the cost.
If the precision for setting the length of the film member composing each gap member and the precision for wrapping the film member to the charging roller “a” are lowered to improve the productivity of the charging roller and to reduce the cost, it is inevitable that a space is generated between the ends of the gap member which is wrapped almost all the way around the charging roller or these ends are superposed on each other in the vertical direction. However, under the aforementioned condition, there is a portion without gap member in the axial direction of the charging roller or a variation in thickness of the gap member at the joint position of the charging roller. When the joint portion comes to a nip portion (contact portion) between the image carrier and the gap member, the charge gap G varies. Consequently, it is impossible to always obtain stable charging of the image carrier.
In the gap member of the charging roller disclosed in Document 1, as shown in FIG. 34B and FIG. 34C, the film member as the gap member “d” is formed to have tilt ends d1, d2 and to have such a length as to form a space “s” between the ends d1, d2 when wrapped around the charging roller “a”. Accordingly, in a state that the film member is wrapped around the charging roller “a”, the gap member “d” exists all the way in the circumferential direction of the charging roller “a” as seen in axial direction of the charging roller “a”. Therefore, the constant charge gap G is maintained even with the joint portion and without strict dimensional control of the film member. The same is true for the film member as the other gap member “e”, but not illustrated.
As shown in FIG. 34D and FIG. 34E, the film member as the gap member “d” is formed to have a length longer than the circumferential length of the charging roller “a” so that the other end portion d2 of the film member is lapped with one end portion d1 of the film member and is shifted in the axial direction of the charging roller “a” when the film member is wrapped around the charging roller “a”. Accordingly, the gap member “d” exists all the way in the circumferential direction of the charging roller “a” as seen in axial direction of the charging roller “a”. Therefore, similarly, the constant charge gap G is maintained without strict dimensional control of the film member. The same is true for the film member as the other gap member “e”, but not illustrated.
Another method for making the gap member “d” to exist all the way in the circumferential direction of the charging roller “a” as seen in the axial direction of the charging roller “a” is also disclosed in Document 1, but the description will be omitted.
However, the charging roller “a” for non-contact charging to be used for an image forming apparatus, disclosed in the aforementioned Document 1 and Document 2, is structured such that the rotary shafts “g”, “h” positioned outside of the pair of gap members “d”, “e” are pressed toward the photoconductive drum “f” by springs (in this specification, a portion between the gap members “d”, “e” is referred to the inside of the gap members “d”, “e” while portions opposite to the inside relative to the gap members “d”, “e” are referred to the outside of the gap members “d”, “e”.). Therefore, as shown in FIG. 35, the contact portions between the gap members “d”, “e” and the photoconductive drum “f” function as fulcrums and portions, to which spring biasing force is applied, of the rotary shafts “g”, “h” outside of the gap members “d”, “e” function as power points so as to cause deflection (bending deformation) Dr of the portion “a1”, positioned inside the gap members “d”, “e”, of the charging roller “a” in a direction apart from the photoconductive drum “f”. Normally, the maximum of deflection Dr of the charging roller “a” is positioned at the middle point in the axial direction between the gap members “d”, “e”.
Since the rotary shaft “i”, “j” coaxially projecting in the axial direction from the both ends of the photoconductive drum “f” are rotatably supported on the apparatus body (not shown) by bearings, the photoconductive drum “f” is pressed by the gap members “d”, “e” so as to cause deflection (bending deformation) Do in a direction apart from the charging roller “a”, i.e. the direction opposite to that of the deflection Dr of the charging roller “a”. Normally, the maximum of deflection Do of the photoconductive drum “f” is positioned at the middle point in the axial direction thereof.
Since the charging roller “a” and the photoconductive drum “f” deflect in the opposite directions, the charge gap G between the charging roller “a” and the photoconductive drum “f” varies in the axial direction, i.e. becomes not constant. Therefore, the uniform charge on the photoconductive drum “f” by the charging roller “a” is impossible. There is a problem that it is difficult to obtain stable charge.
Especially, recently it is more strongly desired to reduce the size and reduce the footprint of image forming apparatuses of electrophotographic type such as a printer of electrophotographic type. Accordingly, process units and function parts inside thereof are required to be smaller and to have high accuracy and it is required to place them optimally. It is therefore required to reduce the sizes of photoconductive drum and charging roller. If the outer diameter or the thickness of the photoconductive drum or the outer diameter of the charging roller is reduced, the aforementioned problem must be bigger.
As the charging roller “a” is driven to rotate directly by driving force of the motor via the driving gear of the photoconductive drum “f” and the power transmission gear train, the charging roller “a” receives pressure from the photoconductive drum “f” in a direction apart from the photoconductive drum “f” so that the charge gap G between the charging roller “a” and the photoconductive drum “f” varies and becomes unstable. Accordingly, the uniform charge on the photoconductive drum “f” by the charging roller “a” in the axial direction is impossible. There is a problem that it is difficult to obtain stable charge. Especially, this problem is significantly bigger in case where the charging roller “a” is composed of a non-elastic member.
If the charging roller “a” is adapted to be not directly driven via the gear train, the charging roller “a” is adapted to be driven to rotate by driving torque of the photoconductive drum “f” which is transmitted to the charging roller “a” by means of friction between the gap members “d”, “e” and the photoconductive drum “f”. However, as the circumferential environment varies or the friction coefficient between the gap member “d”, “e” and the photoconductive drum “f” varies due to adhesion of foreign matter such as toner particles to the gap members “d”, “e”, the driving torque of the photoconductive drum “f” is not effectively transmitted to the charging roller “a” so that the rotation of the charging roller “a” becomes unstable. The unstable rotation of the charging roller “a” causes vibration due to contact between the charging roller “a” and the photoconductive drum “f” so that the charge gap G varies slightly. Especially, in case where the charging roller “a” is composed of a non-elastic member, this vibration may become strongly apparent. This is because the non-elastic charging roller is different from the elastic charging roller made of rubber or the like in that the contact between the charging roller “a” and the photoconductive drum “f” is substantially line contact so that it is impossible to ensure enough nip pressure at the contact between the charging roller “a” and the photoconductive drum “f” and it is therefore difficult to stably drive the charging roller “a” over the long term.
In the image forming apparatus disclosed in Documents 1 and 2, a transfer roller to be in contact with the photoconductive drum is arranged in a region opposite to the charging roller relative to a line which is passing through the center of the photoconductive drum and is perpendicular to a line connecting the center of the photoconductive drum and the center of the charging roller, thereby somewhat preventing the photoconductive drum from being deflected by the pressure from the charging roller as mentioned above.
In the image forming apparatus disclosed in Documents 1 and 2, however, the deflection of the photoconductive drum due to the pressure of the charging roller can not be effectively prevented because the transfer roller is just arranged in the region opposite to the charging roller relative to the perpendicular line. In the image forming apparatus disclosed in Documents 1 and 2, therefore, it is difficult to readily obtain the high-precision charge gap which is uniform in the axial direction.
Further, when the film members as the gap members “d”, “e” are just wrapped around the peripheral surface of the charging roller “a” in the manner as the charging roller disclosed in Document 1, there is a problem that, as the pressure contact between the gap members “d”, “e” and the photoconductive drum is repeated, at least one of the ends of the gap members “d”, “e” unstick and ride up from the photoconductive drum. Especially the end on the side starting the ingress into the nip portion between the gap member “d”, “e” and the photoconductive drum “f” easily unstick because pressing force from the photoconductive drum is repeatedly applied to the aforementioned end at the nip portion in the direction promoting unsticking. In case where the photoconductive drum “f” and the charging roller “a” are stopped from rotating when the portion of the second gap member “e” is positioned at the nip portion between the photoconductive drum “f” and the second gap member “e”, there is the following problem when the portion not projecting outside of the peripheral surface 3s including the rear end of the other end portion 3e2 is in contact with the photoconductive drum “f”. That is, the photoconductive drum “f” and the charging roller “a” rotate at substantially the same circumferential velocity but there is slight differential speed between the circumferential velocity of the photoconductive drum “f” and the circumferential velocity of the charging roller “a” and only the photoconductive drum “f” slightly rotates due to backlash of the gear train for transmitting torque at the moment of the stop of the charging roller “a”. Consequently, it is very rare case, but the other end portion of the gap member “d”, “e” may also unstuck from the charging roller “a”. Further, in case of non-elastic charging roller “a”, the unsticking of the gap members “d”, “e” occurs with increasing frequency.
If the end(s) of the gap members “d”, “e” ride up, the charge gap G by the gap members “d”, “e” varies according to the rotation of the charging roller and can not kept constant. Therefore, it is difficult to conduct uniform and stable charge relative to the photoconductive drum.